Image intensifier tubes are well known in the industry by their commonly used names, based on the generic generation from which their design came into being. The tubes have evolved from Generation 0 to the current Generation III. These tubes have typically been produced in both 18 mm and 25 mm diameter formats.
A significant portion of the military and commercial night vision equipment currently in use was designed to physically accommodate a 25 mm format Generation II (Gen II) image intensifier tube. The military equipment that uses the Gen II tube, includes Individual Served Weapon Sights, Crew Served Weapon Sights, night vision devices that facilitate the operation of motorcraft in low light conditions and for other applications as well.
The Gen II image intensifier tube conforms to detailed U.S. military specifications, and is identified by its U.S. military part number: MX-9644. The performance of the Gen II image intensifier tube is no longer the state of the art. The Gen II image intensifier tube is an inverter tube and exhibits a gain at 6.times.10.sup.-6 foot candles input of from 20,000 to 70,000 with a typical gain of 50,000. The photocathode of a Gen II tube exhibits a luminous sensitivity of approximately 325 microamps per lumen at 2856 K. The Gen II image intensifier tube exhibits a signal-to-noise ratio of approximately 4:1 and a resolution of twenty eight line pairs per millimeter (lp/mm).
A higher performance image intensifier tube has been developed in the Generation III (Gen III) proximity focused image intensifier tube. A Gen III image intensifier tube employs a gallium arsenide photocathode which has an improved photosensitivity that operates at starlight levels and below. A Gen III image intensifier device, with a glass output screen, exhibits a luminous gain in the range of 20,000 to 70,000 at 2.0.times.10 thus -6 foot candles. The sensitivity of the Gen III photocathode is over 1000 microamps per lumen at 2856 K., which is more that three times that of the Gen II tube. The signal-to-noise ratio has been increased to approximately 16:1, and the resolution, to 36-40 lp/mm.
Generation III (Gen III) is a generic term established by the United States Department of the Army at its Night Vision Labs at Fort Bellmore. The Gen III image intensifier tube is identified by Military Part Number MX-10160. The criteria set forth by the Department of the Army in distinguishing a Gen III tube from a MX-9644 Gen II tube is that a Gen III tube utilizes a gallium arsenide photocathode and an ion barrier film over the microchannel plate, while the Gen II tube does not.
In view of the above performance statistics it should be obvious that the Gen III image intensifier tube is more desirable that the Gen II image intensifier tube, and the Gen III tube should be substituted for the Gen II tube wherever possible. However, certain problems arise when such a substitution is attempted. As has been previously mentioned, many night vision applications were designed and built around the Gen II image intensifier tube. In such applications the size and shape of the night vision device was formed to enclose the Gen II tube. Additionally, the objective lens optics and the eyepiece optics were designed to complement the input and output of the Gen II tube.
Additionally, in order to operate properly and form a sharp image across the field of a 25 mm Gen II Electrostatically focusable image intensifier tube, the photocathode surface had to be curved. To accommodate this curvature, the image tube input faceplate was constructed of fiber optics. In 25 mm Gen III wafer tubes, the photocathode can be flat and thus be applied to the inside of a flat glass input plate. Use of a glass plate input faceplate further improves tube sensitivity. In contrast to an electrostatic tube, a wafer tube requires a fiber optic twist/extender for image inversion. Therefore, to make a Gen III retrofit tube for the Gen II electrostatic in the prior art required two modifications to a normal 25 mm wafer tube. First, the Gen III photocathode was placed on a fiber optic input window to accommodate the presently fielded devices' objective lenses (these lenses are not designed to focus through the glass faceplate). Second, a fiber optic twist/extender was added to the output of the wafer tube to invert the image and to make up for the optical and physical length of the Gen II electrostatic tube. These modifications add weight and reduce optical performance. Tubes that use this approach are the ITT Models F4848 and F9860, which meet the performance and physical requirements of the U.S. Government MX-11619 and MX-11620 image intensifier tubes.
Because of the modifications made to the Gen III wafer tube in the prior art to be a form, fit and function retrofit tube, the various devices cannot benefit from the full performance increase offered by Gen III technology. A major shortcoming is that the image formed by the various devices' objective lenses must fall on a fiber optic input faceplate. The fiber optic faceplate reduces the inherently high photocathode sensitivity, while photocathode sensitivity is a major driver for low light level performance. Another shortcoming is that the fiber optic twist/extender must be longer than needed to duplicate the optical and physical length of the Gen II electrostatic tube. This added length of fiber optics adds unnecessary weight to the device.